Line a muffin tin with paper liners. Preheat the oven to 375 degrees F.
To make the muffin batter use a large bowl and mix together the butter and brown sugar with a hand mixer or stand mixer. Mix for 2 minutes.
Add the egg to the butter and sugar mixture and mix until combined.
Add the mashed banana, greek yogurt and vanilla extract to the butter mixture. Mix until combined.
In a medium bowl whisk the dry ingredients including the flour, baking soda, baking powder, salt and cinnamon. Add the dry ingredients slowly into the wet ingredients and mix until just combined.
To make the crumble topping combine the flour, brown sugar, butter and cinnamon in a small bowl. Use your fingers to mix until the butter is the size of a pea.
Add the batter to your muffin tin and top with the crumble topping. Bake for 20 minutes or until the muffins are lightly browned and a toothpick inserted in the center comes out clean.
Allow to cool before serving.